Measuring Suction and Vacuum Pressure

Explanations for technical queries relating to measuring negative gauge and absolute pressures in the vacuum range.

How is suction pressure measured?
Use a gauge reference pressure measuring device with a range scaled in the negative direction.   The zero to full scale range will depend on the maximum suction pressure expected.

How is a vacuum measured?
A vacuum is any pressure that is below atmospheric pressure.  A vacuum can be measured using an absolute pressure sensing device.  Zero absolute pressure is a perfect vacuum, the higher the absolute pressure the lower the vacuum.

Can a differential range be used for measuring suction pressure?
Yes, there are two ways of using differential pressure sensing instruments (DP) for measuring suction pressure:
  1. Leave the negative side port open to atmosphere and connect the positive side to the suction pressure. The DP should be scaled and calibrated to read in the negative direction.
  2. Leave the positive side open to atmosphere and connect the negative side to the suction pressure so that the DP will measure the difference as if it were a positive pressure.

Is zero bar absolute the same as minus 1 bar gauge?
No, only when atmospheric pressure happens to correspond exactly to 1 bar absolute which is not the case for most of the time.

Is a minus 1 bar range adequate for measuring over the vacuum range?
For most applications yes, especially if measuring low suction pressures.  If measuring a very high vacuum to some degree of accuracy it is best to measure with a positive absolute range since a negative gauge device will give different readings due to the changes in atmospheric pressure.

What type of sensing technology is used for measuring suction pressure?
For suction pressures in the range of 0-10 mbar (~0.15 psi) up to
0-1000 mbar (~15psi) it is possible to use strain gauge diaphragm.  For ranges 0-0.1 mbar up to 0-10 mbar the sensitivity is too small and errors are too great to use a strain gauge diaphragm, so for very low pressures a more sensitive technology such as variable capacitance, inductive or LVDT is used.

What type of sensing technology is used for measuring in the vacuum range?
For vacuum ranges 0-100 mbar up to 0-1000 mbar absolute it is usual to use a lower cost strain gauge diaphragm type.  For ranges from 0-1 mbar up to 0-1000 mbar absolute, capacitive or inductive sensing techniques are the typical methods for measuring higher vacuums more precisely and with less drift.   For ultra high vacuum ranges below 0-1 mbar it is not possible to measure this low a pressure with electromechanical devices and it is necessary to use less direct methods such as thermal conductivity and ionisation techniques.

How do you calibrate the zero reading for an absolute pressure measuring device?
In most situations it is not possible to generate a high enough vacuum due to the limitations of the available vacuum pump, so instead the zero reading must be extrapolated.  To avoid introducing span errors the vacuum should be less than 10% of the vacuum range being calibrated.

What pressure units are used for measuring vacuum pressures?
  • Low vacuum: psi, mbar, kPa, inHg and mmHg
  • High vacuum: Torr, Pa, mmHg, mbar

What happens inside a pressure transducer when negative pressure is applied?
Before negative pressure is applied, there is an equal pressure on the front and rear side of the sensor diaphragm.  When the negative pressure is applied, the rear side pressure is now higher than the front side pressure.  This causes the sensing diaphragm to flex in the reverse direction resulting in a negative output signal.  The negative output signal is often converted to a standardised positive signal such as 4-20mA, 0-10V or 0-5V. 

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17 comments:

  1. If value of vacuum is given as 5.3 KPa or 40 mm of mercury ,How can we come to know whether it is gauge pressure value or absolute pressure value?
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  2. Re: How to know whether a given vacuum value is absolute or gauge reference?

    Its not possible to be certain if you don't know whether the reference point is atmospheric or a perfect vacuum. The correct way to describe a measurement of Vacuum is with absolute pressure but in the real world it is often used to describe a suction pressure as well.

    5.3 kPa is a low value relative to atmosphere which is ~ 100 kPa, so maybe you can question the application and make a calculated guess on whether it makes sense for it to be close to atmospheric pressure or closer to a perfect vacuum.
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  4. Erika, mmHg stands for millimetres of mercury pressure units and represents the hydrostatic pressure generated by a 1 millimetre high column of liquid mercury.
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  5. The suction pressure measured at the point before the vacuum pump as 33.8 mmHg. How do we calculate the absolute pressure and does it indicate the value is gauge pressure?
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  6. The vacuum range is ~ 0 to 760 mmHg absolute, so if you have pumping a closed airtight volume for a reasonable period of time, then 33.8 mmHg is most likely an absolute pressure, since it would not be a very powerful vacuum pump if it was only generating minus 33.8 mmHg gauge pressure (suction).

    If the volume being pumped is not closed and the gas is being replenished at a high rate, then 33.8 mmHg might be a negative gauge pressure, since the vacuum pump maybe too small and will never achieve a high vacuum.

    What happens to the pressure if you turn off the vacuum pump and slowly vent the system? If the pressure rises to around 760 mmHg then you are reading in absolute, but if it reduces to around 0 mmHg then it is measuring negative gauge or suction pressure.

    To convert absolute pressure to gauge pressure you will need to mmeasure the atmospheric pressure and take it away from the vacuum reading e.g. 33.8 mmHg absolute with an atmospheric pressure of 760 mmHg will be -726.2 mmHg gauge. Remember atmospheric pressure is always change, so even if your vacuum reading is stable, the negative gauge reading will need to be constantly updated.

    To convert gauge pressure into absolute pressure you will need to know the atmospheric pressure, but since in this case the measuring device is negative gauge, you will need an additional barometer to measure the atmospheric pressure. This can be then added to the suction pressure to determine the absolute pressure of the vacuum, e.g. a barometric reading of 740 mmHg added to -33.8 mmHg gauge is 706.2 mmHg absolute. Again remember since atmospheric pressure is always changing the converted absolute pressure will also change, even if the suction pressure does not.
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  8. i got confused where we r only able to create only 760 mmhg as vacuum then how can we create such a high vaccum around 1555 mmhg with an aspiraor
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  9. Venni, a vacuum pressure is any pressure from absolute zero pressure up to atmospheric pressure which is approx 760mmHg absolute, higher or lower depending on weather conditions and altitude. A high vacuum is actually a very low pressure in the range 1×10−3 to 1×10−9 mmHg absolute.

    If you are referring to suction pressure you might find this post on suction pressure greater than atmospheric pressure more relevant to your question.
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  10. hi s1 ,then i have a question, about tail gas analyzers in sulphur recovery process where clauss process is applied , model ametek 920. an aspirator is used to suck the sample through the measuring cell the length of mearing cell is 30 cm, but the pressure of the measuring cell is displayed as 890 mmhg, is it possible
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  11. Venni, 890mmHg is ~1187 mbar, if this is an absolute pressure then your measuring cell is probably at a higher pressure than atmospheric. This is presumably possible if you have a higher gas pressure at the other end of the measuring cell. We are not familiar with the aspirator or the gas analyser so we would recommend you consult with the manufacturer for specific details.
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  12. i tried keeping mails to them (provider) but no resonce from them am searching for the content from the last ten days ., plz do me a favour for me cn u plz send any links of forums so that i can discuss with some other people ., thanking you
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  13. Sorry, we can't help you with a suitable forum, but in case you haven't found it already here's the product page for the Ametek 920 Analyser which includes several documents.
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  14. thank you so much dear, the link is useful to me upto some extent
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  15. This my sound like a strange question but is it possible to create a vacuum in a large enough container to suck up something moving, i have been trying to find mathematical formulas for creating such a vacuum and have had no luck if you can point me in the right direction in would be much appreciated.
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  16. Hi S1, I preparing to test a centrifugal pump test to compare with the manufacture data. I plan to put 2 pressure gauges one connected to the suction and the other to the discharge. How will I be able to record a negative value of the suction pressure if the pressure gauge does not have negative readings?
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  17. Re: preparing to test a centrifugal pump test

    If you are defining the suction pressure as the difference between the suction and discharge pressure, your ability to measure it will depend on the following:

    1) Whether you are using a gauge (measures 0 when vented) or absolute (measures ~1000mbar when vented) reference pressure gauge.

    2) If at least one of the pressures is below ambient pressure.

    If the pressure gauges have a gauge reference then without a negative scale, you will only be able to measure the difference if both pressures are always above atmospheric pressure.

    If the pressure gauges have an absolute reference then you will be able to measure the difference at any pressure as long as it is within the range of the pressure gauges.
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